Power Plant Engineering

HYDRO-ELECTRIC POWER PLANTS 353

or between medium head and low head. The power plant can be classified on the basis of head roughly
in the following manner:
(a) High head plants. About 100 m and above.
(b) Medium head plants. about 30 to 500 m.
(c) Low head plants. Upto about 50 m.
High Head Plants. Fig. 11.6 shows the elevation of a high head plant. The water is taken from
the reservoir through tunnels which distribute the water to penstock through which the water is con-
veyed to the turbines. Alternately, the water from the reservoir can be taken to a smaller storage known
as a forebay, by mans of tunnels. From the forebay, the water is then distributed to the penstocks. The
function of the forebay is to distribute the water to penstocks leading to turbines. The inflow to the
forebay is so regulated that the level in the forebay remains nearly constant. The turbines will thus be fed
with under a constant static head. Thus, the forebays help to regulate the demand for water according to
the load on the turbines. Trash racks are fitted at the inlets of the tunnels to prevent the foreign matter
from going into the tunnels. In places; where it in not possible to construct forebays, vertical construc-
tions known as `surge tanks' are built. The surge tanks are provided before the valve house and after the
tunnel from the head works. The function of the surge tank is to prevent a sudden pressure rise in the
penstock when the load on the turbines decreases and the inlet valves to the turbines are suddenly
closed. In the valve house, the butterfly valves or the sluice type valves control the water flow in the
penstocks and these valves are electrically driven. Gate valves are also there in the power house to
control the water flow through the turbines. after flowing through tile turbines. The water is discharged
to the tail race.

Reservoir

Head Race

Trash Rack

Tunnel

Surge Tank

Valve House

Penstock

Power

House Ta i lRace

Fig. 11.6

Low Head Power Plants. These power plants are also known as Canal power plants. Such a

plant is shown in Fig. 11.7.

A dam is built on the river and the water is diverted into a canal which conveys the water into a

forebay from where the water is allowed to flow through turbines. After this, the water is again dis-

charged into the river through a tail race. At the mouth of the canal, head gates are fitted to control the

flow in the canal. Before the water enters the turbines from the forebay. It is made to flow through

screens or trash-racks so that no suspended matter goes into the turbines. If there is any excess water due

to increased flow in the river or due to decrease of load on the plant, it will flow over the top of the dam

or a waste weir can be constructed along the forebay so that the excess water flows over it into the river.

For periodic cleaning and repair of the canal and the forebay, a drain gate is provide on the side of the

waste weir. The head gate is closed and the drain gate is opened so that whole of the water is drawn from

the forebay and the canal for their cleaning and repair.